I had this happen to me in Thailand once. Never done bareback with a WG before, she was doing OWO and kept sliding up and down. Had a moment to stop her when I realised she was going to mount me bareback but was thinking with the small head instead of the big one.
Got lucky, came away clean but was worried for ages afterwards. Best bet is to get down to a Gum clinic and get yourself checked out, you'll save yourself a lot of time worrying. Give a false name if you want to stay anonymous.